Electron surface tunneling is a well known phenomenon. It is predicted by quantum mechanical theory, and is exploited in surface tunneling microscopes (STM) capable of distinguishing individual atoms on surfaces. The quantum theory of surface tunneling focuses on the possibility that an electron can jump from the electron cloud on the surface of one material to an electron cloud on the surface of another material. An important feature is that the two materials are physically separated by a “forbidden” region in which free electrons are not allowed to exist. Examples of materials for such a forbidden region are electrical insulators, a vacuum, and dry air. An electron can only survive for a very short time in the “forbidden” region. If an electron makes it across the region, it is said to have “tunneled” through the region.
A basic prior art experiment 10 which demonstrates surface tunneling is shown in FIG. 1. In this experiment, there is a conducting surface 11 and a conducting tip 12, which is brought into very close proximity to the conducting surface 11. An electrical potential difference v is applied between the tip 12 and surface 11, which creates an electrical potential difference across a forbidden region 14. The potential difference helps increase the chance that an electron 13 in the tip 12 can make the jump across region 14 to the surface 11. The tunneling of the electrons 13 gives rise to an electrical current i between the tip 12 and surface 11 called “the tunnel current”. To understand what is happening in this experiment, one must use quantum theory to find wave function solutions that satisfy Schrödinger's equation with the boundary conditions for the three regions (i.e., tip, forbidden region, and surface). If the Wentzel, Kramer, and Brillouin (“WKB”) approximation is used, which makes certain simplified assumptions about the wave function solutions, and if it is further assumed that the tip 12 and surface 11 are made of the same material, and that the electrons 13 are distributed according to the Fermi statistics, Simmons formalism can be used to derive a tunneling current density given by:
                              J          tunnel                =                              (                          e                              4                ⁢                                  π                  2                                ⁢                ℏ                ⁢                                                                  ⁢                                  d                  2                                                      )                    ⁢                                    {                                                                    (                                                                  Φ                        0                                            -                                              eV                        2                                                              )                                    ⁢                                      exp                    ⁡                                          [                                                                        -                                                                                    2                              ⁢                              d                                                        ℏ                                                                          ⁢                                                                              (                                                          2                              ⁢                                                                                                m                                  e                                                                ⁡                                                                  (                                                                                                            Φ                                      0                                                                        -                                                                          eV                                      2                                                                                                        )                                                                                                                      )                                                                                1                            2                                                                                              ]                                                                      -                                                      (                                                                  Φ                        0                                            +                                              eV                        2                                                              )                                    ⁢                                      exp                    ⁡                                          [                                                                        -                                                                                    2                              ⁢                              d                                                        ℏ                                                                          ⁢                                                                              (                                                          2                              ⁢                                                                                                m                                  e                                                                ⁡                                                                  (                                                                                                            Φ                                      0                                                                        +                                                                          eV                                      2                                                                                                        )                                                                                                                      )                                                                                1                            2                                                                                              ]                                                                                  }                        .                                              (        1        )            
It is important to realize from equation (1) that there is an exponential dependence between the tunneling current i and the distance d from the tip 12 to the surface 11. Therefore, even minute changes in distance d will lead to a significant change in the tunneling current i. In FIG. 2, the dependence of the tunneling current i on the distance d is shown for the prior art experiment of FIG. 1 with a gold tip 12 and surface 11, an electrical potential of 2 V, and an assumed tip area of 20 nm2. As can be seen in FIG. 2, the tip 12 must be brought very close to the surface 11 to achieve a measurable tunnel current; however, even a change of distance d of 1 Å (less than half the diameter of an atom) will change the tunneling current i by a factor of 10.
Bringing the tip 12 in such close proximity to the surface 11 and maintaining its distance d without touching the surface 11 presents a tremendous control problem. A large scale “equivalent” of this control problem would be to drive a car at 60 mph up to a wall and stopping without hitting the wall, such that the bumper is less than 0.1″ from the wall. With the use of micro electro mechanical systems (MEMS) technology, it has become possible to realize prior art devices, such as device 20, shown in FIG. 3, in which a very sharp tip 21 is attached to a suspension cantilever 22 with built-in actuator 23 that can move the tip 21 with extremely small amplitudes. The tip 21 and cantilever 22 are normally attached to a larger structure 24 that can be moved with conventional actuators to bring the tip 21 within about 1 micron of the surface 25 (˜2000 times larger than the needed distance). The actuators 23 on the cantilever 22 are then engaged, while constantly monitoring the tunnel current i, until the specified tunnel current is achieved.
One approach for realizing a microphone 30 using a tunneling tip 31 is shown in FIG. 4. In this case, MEMS technology is used to fabricate a sensitive membrane 35, which will deflect due to an acoustic sound pressure incident on membrane 35. By using MEMS technology for the assembly, a structure 34 with a few microns initial distance between the membrane 35 and the tip 31 can be realized, which means only the actuators 33 of cantilever 32 are needed to control the tip movement. The control circuit of the actuator 33 is used in a feedback loop to maintain a certain tunnel current, and as the membrane 35 deflects, the actuator signal is changed to maintain the tunnel current, and hence the tip distance. The actuator signal therefore becomes the microphone output signal of microphone 30.
There are a number of problems with this basic structure. First, the fabrication of such a MEMS structure is very complicated and difficult to realize. The result would be that the cost of the device would be exceedingly high when compared to other microphone technologies. Second, the cantilever 32 will have a significant sensitivity to vibration, due to its inertial mass, which will manifest itself as an artifact in the microphone signal. The vibration sensitivity will be much higher for this structure than other comparable microphone structures based on other detection methods (e.g., piezoelectric or capacitive). In addition, the resonance frequency of the cantilever tip 31 is bound to fall within the frequency range of interest in the microphone 30, which will make control of the tip deflection extremely difficult or impossible.
It is therefore an object of the present invention to realize a novel structure based on MEMS technology, in which the fabrication of a tunneling tip and pressure sensitive membrane is integrated to lower the fabrication cost of the device.
It is another object of the present invention to reduce the vibration sensitivity of the tunneling microphone to a level comparable to other MEMS microphone detection technologies.
It is a further object of the present invention to design the tunneling microphone structure such that a wide acoustic bandwidth can be achieved.